Answer: It sounds as though you've allowed some suckers to grow from the roots and the suckers are crowding the trunk of your tree. Crape Myrtles often send out suckers so you'll need to make removing them part of your regular routine. I find it easiest to dig around the base of the tree to expose the places where the suckers are emerging then pull them off the roots. This way you'll take the growth cells along with the sucker and that should keep that area from producing a new sucker. If you simply cut the suckers off at ground level they will regrow. After removing all the suckers, put the soil back over the crown and roots of the plant and watering down well.
Best wishes with your landscape!
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